Answer

Memory management in Perl is automated at the interpreter level. Historically, Perl was created as a language where the programmer should not have to manage memory explicitly, allowing them to focus on algorithms. Resource deallocation occurs automatically, but vital details about the mechanism and its limitations should be known by every developer.

Background:

From the first versions of Perl, the language developers chose an approach where memory is allocated dynamically and returned back to the system when there are no more references to the object. This is called reference counting.

Issue:

The main nuance is that the mechanism does not recognize cyclic references. If two structures reference each other, neither reaches "zero" in the reference count, and the memory is not freed.

Solution:

Perl uses a built-in reference counter for each object and variable. When the counter drops to zero, the memory is automatically freed. To combat cyclic references, it is recommended to use the Scalar::Util::weaken module to create "weak" references that do not increase counters or manually break the cycles.

Code example:

use Scalar::Util 'weaken';

my $a = {};
my $b = { parent => $a };
$a->{child} = $b;
weaken($a->{child});

Key features:

Automatic memory deallocation when reference counter reaches zero.
Vulnerability to cyclic references (memory leak).
Additional modules (Scalar::Util::weaken, Devel::Cycle) for reference control.

Trick questions.

Can Perl automatically remove cyclic references with garbage collection like Java does?

No. The standard implementation of Perl 5 does not have a full-fledged garbage collector, only reference counting. Cyclic references can only be freed manually.

What happens to the memory of a variable if you use undef on a scalar or anonymous structure?

The undef operator decreases the reference count. If no other references remain, the memory will be freed. But if there are still references (for example, from other structures), the object will remain in memory.

my $a = [];
my $b = $a;
undef $a; # $b still references — memory not freed

If a variable goes out of scope, is memory always freed?

No, if the object is participating in a cyclic reference or there exists a global reference, memory will not be freed until all external links are removed.

Common mistakes and anti-patterns

Unintentional creation of cyclic references within complex structures — leads to memory leaks.
Storing large temporary objects in global variables — prevents memory from being freed.

Real-life example

Negative case

Storing a directory tree where each node holds a reference to its parent and children. We do not use weak references. Memory is not freed until the program ends.

Pros:

Regular references are easy to implement

Cons:

Significant memory leak during long runs

Positive case

We use Scalar::Util::weaken for the parent reference so that the reference does not increase the counter, and memory is allocated precisely as needed.